1. Field of the Invention
This invention relates to devices for forming gels for electrophoresis, and for performing electrophoresis using those gels. More specifically, the invention relates to devices for facilitating electrophoresis in horizontally oriented gels.
2. Related Art
Electrophoretic separation of nucleic acids in agarose gels is an important technique which is commonly used in molecular biology research and genetic diagnosis.
Many devices have been designed to facilitate gel electrophoresis of biologically significant macromolecules. Some of these devices are designed so as to orient the gel vertically. Other devices are designed to orient the gel horizontally.
A vertical orientation for the gel was generally preferred for the electrophoresis of nucleic acids in such applications as nucleic acid sequencing. The disadvantages of devices employing vertical orientation of the gel caused them to be replaced for many other applications by devices in which agarose gels were cast on a horizontal surface so that the electrophoretic separation occurred in a substantially horizontal direction. To case a horizontal gel, one must seal the ends of a gel casting deck onto which the heated molten gel material is poured. The casting deck thus provides a shallow cavity in which the molten agarose can cool and gel. Furthermore, to run an agarose gel, it is necessary to have effective and uniform electrical contact between the gel and the electrophoresis buffer at the ends of the gel. Several known devices have been used to cast horizontal agarose gels.
A first approach to casting horizontal gels makes use of "wicks" of agarose case in chambers at each end of the platform upon which "separation" agarose gel is later cast. The wicks seal in the molten agarose during the casting procedure. The wicks provide an effective electrical connection between the separation gel and buffer reservoirs located on the outside of the respective wicks. An apparatus which typifies this designs disclosed in U.S. Pat. No. 4,234,400 to Kaplan et al.
Another approach adopted by designers of horizontal gel electrophoresis devices employs a removable tray (or deck) upon which the agarose gel could be cast. Certain of these devices cast a gel in an apparatus separate from that used to carry out the actual electrophoresis. Typically, the deck in which the agarose gel is cast consists of two opposing sides and two open ends which, when the gel is being used to electrophoretically separate macromolecules, will be directly exposed to the buffer. Commonly, this deck is situated between two reservoirs which contain a buffered salt solution. Electrodes are installed into the respective reservoirs to provide electrical current from a power supply. The two open ends of the deck allow effective and uniform electrical contact between the gel and the buffer. To cast the gel in the first place, however, it is necessary to seal these open ends of the deck to create a fluid-tight cavity for containing the molten agarose solution before it cooled and gelled.
To use this gel after hardening, the seals at the ends of the deck first have to be removed. A first method used to block the open ends of the gel casting deck during the gel cooling and hardening process consists of sealing the ends of the deck with an adhesive tape. The tape is peeled off before installing the gel in the electrophoresis apparatus. Unfortunately, the removal of the tape is time consuming and requires that the gel be manually picked up. Any such manipulation or other disturbance to the gel is undesirable, since agarose gels are fragile.
A second method of blocking the open ends of the casting deck during the gelation process consists of placing rectangular blocks into slots cut into the side walls of the electrophoresis apparatus. These blocks are thereby positioned against the open ends of the deck, providing a surface which seals the open ends of the deck during the casting. Unfortunately, to achieve a reliably tight seal against the ends of the deck is difficult, since the reliability of the seal in this technique is dependent on the exact positioning of the side wall slots and the exact size and shape of the rectangular blocks. This rectangular block design requires that the device be constructed with very tight manufacturing tolerances which have to be maintained throughout the life of the device. Such tight tolerance requirements cause increased manufacturing costs.
A simple and convenient means for sealing and unsealing the ends of gel casting decks is therefore desirable.
Frequently, when agarose gels of the sort described above are used to electrophorese DNA, the gel breaks free from the casting deck, possibly as the result of an accidental jostle by laboratory personnel. Having broken free from the casting deck, the gel tends to drift up off the casting deck, supported by the liquid buffer, toward one of the buffer reservoirs. If the gel begins to float toward either buffer reservoir, the electrophoresis of the DNA becomes aberrant or unpredictable, and the value of the electrophoretic analysis is accordingly reduced or lost.
It is therefore desirable to have a casting deck which substantially impedes the gel from dislodging from its casting deck and floating toward a buffer reservoir.
Another problem present in known electrophoresis devices relates to the difficulty in referencing the "combs" which are used to create "wells" in the agarose gel into which samples are placed. Many known devices are designed so that, when the teeth of the combs are inserted into the molten agarose, the combs rest on surfaces which are physically remote from the casting deck. This remote referencing results in nonuniform and inaccurate separation of the ends of the teeth from the floor of the casting deck due to unreliable fitting of adjacent parts in the apparatus. This nonuniformity and inaccuracy in the placement of the combed teeth results in the creation of wells in the gel which are commensurately nonuniform and inaccurate. At times, the teeth of combs in known devices actually contact the floor of the casting deck, so that there is no agarose seal between the well and the casting deck after the agarose had gelled. This missing agarose gel seal sometimes destroys otherwise valuable experimental results.
It is therefore desirable to have a comb reference surface which substantially guarantees the uniformity and accuracy of the depth of wells in agarose gels, and thereby ensures the presence of agarose seals at the bottom of those wells. Such a reference surface would reduce the manufacturing tolerances needed to manufacture the apparatus.
Still another problem in known devices is the danger to which laboratory personnel are exposed. Curiosity or carelessness on the part of the laboratory personnel results in injury from the high voltages involved in electrophoresis and from radioactive or otherwise dangerous chemical reactants.
It is therefore desirable to design electrophoresis devices so that, in operation, the ability of curious or careless laboratory personnel to cause injury to themselves or others is substantially reduced or eliminated.